Extend and document the "wait" command

The wait command was currently implemented but
undocumented and only used internally. This change
adds a second option toi (optionally) override the default
timeout of 5 seconds.

The wait command can be used to wait for a file to appear
during the boot process, for example a disk or other device,
and is a better alternative than an unconditional sleep.

Change-Id: Ia11d8419c0ea84deec0e1bf5adfca5bc76ff38b7
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>
diff --git a/init/builtins.c b/init/builtins.c
index ac9585e..6d3ed25 100644
--- a/init/builtins.c
+++ b/init/builtins.c
@@ -813,6 +813,8 @@
 {
     if (nargs == 2) {
         return wait_for_file(args[1], COMMAND_RETRY_TIMEOUT);
-    }
-    return -1;
+    } else if (nargs == 3) {
+        return wait_for_file(args[1], atoi(args[2]));
+    } else
+        return -1;
 }